Diesel particulates, their effect and control, are at the center of much concern and controversy. Their chemistry and environmental impact present complex issues. Very generally, diesel particulate matter is principally solid particles of carbon and metal compounds with adsorbed hydrocarbons, sulfates and aqueous species. Among the adsorbed species are aldehydes and polycyclic aromatic hydrocarbons (also called PAH's). Some of these organics, as well as the particulates themselves, have been reported to be potential carcinogens. Unburned hydrocarbons are related to the characteristic diesel odor and include aldehydes such as formaldehyde and acrolein. The aldehydes, like the carbon monoxide, are the products of incomplete combustion.
It is not just these organics which are of concern, all particulates are subject to question. In one study, diesel particulates were tested along side TiO.sub.2 and carbon without any adsorbed hydrocarbons. (U. Heinrich, et al, "Tierexperimentelle Inhalationsstudien Zur Frage der Tumorinduzierenden Wirkung von Dieselmotorabgasen und zwei Teststauben", Oklolgische Forschung BMFT/GSF, Munich, 1992) The reporters determined that all species tested showed carcinogenic tendency. Until further work clarifies this matter, it would be prudent to look for systems which could control all particulates--regardless of composition.
Unfortunately, increasing the recovery of particulates simply by modifying trap design or size would increase the rate of back pressure buildup within the trap. Moreover, the various pollutants seem to be interrelated. With reduction of one sometimes increasing levels of another. By modifying combustion to achieve more complete oxidation, decreases can be achieved for pollutants resulting from incomplete combustion, but NO.sub.x is typically increased under these conditions.
NO.sub.x, principally NO and NO.sub.2, contributes to smog, ground level ozone formation and acid rain. NO is produced in large quantities at the high combustion temperatures associated with diesel engines. The NO.sub.2 is formed principally by the post oxidation of NO in the diesel exhaust stream. Several attempts have been made to reduce NO.sub.x, such as by retarding engine timing, exhaust gas recirculation, and the like; however, with current technology, there is a tradeoff between NO.sub.x and particulates. For example, exhaust gas recirculation and engine timing changes can reduce the temperature of combustion to thereby decrease NO.sub.x formation, but combustion is also affected. When NO.sub.x is reduced by these techniques, particulate emissions tend to increase. And, as noted, conditions favoring low emissions of NO.sub.x often favor production of increased levels of CO and HC.
Traps are reasonably effective for controlling particulates, but uncatalyzed traps increase carbon monoxide and catalyzed traps increase the discharge of SO.sub.3 (adding to the weight of particulates) and suffer from other problems. Traps, of course, don't reduce NO.sub.x and efforts made to control NO.sub.x must be carefully selected or the result might be to further increase particulates or other products of incomplete combustion.
Catalyzed diesel traps are not to be equated with triple-effect catalytic converters of the type used for gasoline engines. Triple-effect catalytic converters of this type simply don't work for diesel engines due to the different manner of operation and the different composition of exhaust gases. Reference is made to the following patent publications related to catalyst technology for gasoline engines: U.S. Pat. No. 5,387,569, U.S. Pat. No. 5,386,690, U.S. Pat. No. 5,322,671, WO 94/22983, WO 94/22577 and WO 94/09431. Thus, while these patent publications indicate that catalyst metals can be introduced into gasoline fuel or the combustion air for it, this technology is not the answer to the problems discussed above with regard to the tradeoffs encountered when dealing with the polluting emissions from diesel engines.
The use of diesel traps and the need to improve them has resulted in a great deal of research and a great number of patents and technical publications. The traps are typically constructed of metal or ceramic and are capable of collecting the particulates from the exhaust and withstanding the heat produced by oxidation of carbonaceous deposits which must be burned off at regular intervals.
This burning off, or regeneration, could occur by itself if the operating temperature of the trap were sufficiently high. However, in the typical situation, the exhaust temperature is not constantly high enough, and secondary measures such as electrically heating to raise the trap temperature, using a catalyst to reduce the combustion temperature, supplemental burners, and exhaust gas throttling have been attempted. Electrical heaters create intense loads on batteries, most needed at lower power settings where the electrical output is also low. Supplemental heaters and exhaust gas throttling can lower efficiency.
The uses of exhaust gas oxidation catalysts and catalyzed diesel particulate traps to enhance burn-off or regeneration, have taken many forms, but none has been found to be fully satisfactory. While exhaust gas oxidation catalysts can be very effective in reducing carbon monoxide and unburned hydrocarbons, they are either too easily fouled, catalyze the oxidation of SO.sub.2 to SO.sub.3 (which then combines with water and increases the weight of particulates), typically burn off only the soluble organic fraction of the particulates, or have two more of these shortcomings. No catalytic device is known which can collect particulates and burn them off at a practical low temperature while reducing oxidation of SO.sub.2 to SO.sub.3 and also decreasing the emissions of gaseous hydrocarbons and carbon monoxide.
In "A New Generation of Diesel Oxidation Catalysts", Society of Automotive Engineers (SAE) Paper No. 922330, 1992, R. Beckman, et al., assert that the technical challenge is to find a catalyst which selectively catalyzes the oxidation of carbonaceous components at low exhaust temperatures typical of diesels operating at partial load, and does not oxidize sulfur dioxide or nitrogen oxide at high load temperatures. They described tests studying the aging of platinum-catalyzed cordierite honeycomb traps, and concluded, inter alla, that the aging was related to adsorption of sulfur and that this depended on both the sulfur content of the fuel and the phosphorous content of the lubricating oil. With control of both of these, aging could be slowed. However, sulfur will remain in diesel fuels, even with planned reduction to 0.05%, and there will remain a need for a means to maintain the activity of catalysts for reducing emissions of carbon monoxide and unburned hydrocarbons, and reducing the ignition temperature of loaded traps. There is no indication that the combined use of mechanical devices and fuel additives could either: (1) reduce NO.sub.x emissions and particulates simultaneously; or (2) reduce the temperature necessary for regenerating a particulate trap while also reducing the emissions of carbon monoxide and unburned hydrocarbons.
In "Control of Diesel Engine Exhaust Emissions in Underground Mining", 2nd U.S. Mine Ventilation Symposium, Reno, Nevada, Sep. 23-25, 1985, at page 637, S. Snider and J. J. Stekar report that precious metal catalysts in a catalytic trap oxidizer and a "catalyzed Corning trap" were effective in the capture of particulate matter, but both systems increased the conversion of SO.sub.2 to SO.sub.3. The increase in the rate of oxidation of the benign, gaseous dioxide form to the trioxide form results in the adsorption of greater amounts of acid sulfates and associated water onto discharged particulates. Thus, the weight of the particulates is increased, and the difficulty in reaching regulatory compliance is increased.
The Snider, et al., report also discussed several other approaches, including the use of a fuel additive containing 80 ppm manganese and 20 ppm copper to reduce the regeneration temperature of the trap. While this was effective in reducing the particulate ignition temperature, no measurable reductions in carbon monoxide, unburned hydrocarbons or NO.sub.x were noted. Moreover, where Snider, et al., also indicate that precious metal catalysts could be expected to increase the oxidation of SO.sub.2 to SO.sub.3, these measures do not address an overall solution to the diesel emission problem.
A number of fuel additives have been proposed for adding to diesel fuels to affect the nature of particulates deposited on diesel traps or otherwise improve the collection or disposition of the particulates. For example, U.S. Pat. Nos. 5,360,459 and 5,374,154, describe the use of copper-containing organometallic complexes which, when added to diesel fuel, tend to reduce the ignition temperature of exhaust particulates. Again, these additives do not lower carbon monoxide and unburned hydrocarbons. In another similar teaching, U.S. Pat. No. 4,458,357 describes the use of a fuel additive containing cerium and manganese to reduce the quantity of particulate material necessary to sustain combustion of the particulates on the trap--once combustion is initiated by a glow plug. In U.S. Pat. No. 5,034,020, fuel-soluble platinum additives are disclosed to provide or replenish catalyst metals on a diesel trap to facilitate burning off of trapped particulates. In U.S. Pat. No. 5,322,671, a catalyst comprising platinum, rhodium or rhenium is added directly to a special catalyst chamber, meant to replace a conventional gasoline engine three-way catalytic converter. This patent does not address the issue of particulate emissions from diesel engines. And, again, none of these patents address an overall solution to the diesel emission problem.
In "Assessment of Diesel Particulate Control--Direct and Catalytic Oxidation", Society of Automotive Engineers (SAE) Paper No. 81 0112, 1981, Murphy, Hillenbrand, Trayser, and Wasser have reported that the addition of catalyst metal to trapped particulates can decrease the particulate ignition temperatures. Neither this disclosure nor any of the others above, however, indicate or suggest specific combinations of mechanical means and fuel additives that can reduce particulates and NO.sub.x at the same time, or a combination of fuel additives which can reduce the balance point temperature of the particulates trapped on a diesel trap significantly while also significantly reducing emissions of unburned hydrocarbons and carbon monoxide. Especially, none of these disclosures indicate any recognition that catalysts can be effective for these purposes and also provide, over time, the selective oxidation of fuel sulfur to SO.sub.2 without inactivation of the catalyst.
In a 1987 report on catalyzed traps, R. W. McCabe and R. M. Sinkevitch summarized their studies of diesel traps catalyzed with platinum and lithium, both individually and in combination. (Oxidation of Diesel Particulates by Catalyzed Wall-Flow Monolith Filters. 2. Regeneration Characteristics of Platinum, Lithium, and Platinum-Lithium Catalyzed Filters; SAE Technical Paper Series-872137) They noted that carbon monoxide conversion to the dioxide was negligible over the lithium filter, good for platinum, but good only initially for the combined catalyst. They further noted that platinum undergoes a reversible inhibition due to the presence of SO.sub.2, but in the presence of the lithium catalyst there is apparently a wetting of the platinum crystallites by Li.sub.2 O.sub.2. From this work, it can be seen that platinum and lithium on their own help burn particulates at low temperature, but not necessarily low enough to make supplemental heat unnecessary.
In a more recent report, B. Krutzsch and G. Wenninger discussed their investigation of sodium and lithium-based fuel additives. (Effect of Sodium- and Lithium-Based Fuel Additives on the Regeneration Efficiency of Diesel Particulate Filters, SAE Technical Paper Series 922188, 1992) They noted that the predominantly used diesel additives were based on transition metals such as iron, copper, and manganese. The transition metals were seen to form oxides which foul the traps and cannot be easily removed. They found that the sodium and lithium additives permitted regeneration at temperatures low enough to possibly eliminate the need for supplementary heat, and did, therefore, have some promise in improving trap operation as was achieved previously with the transition metal catalysts. However, they also pointed out that there was no effect on the gaseous components, thus both carbon monoxide and unburned hydrocarbon levels remained higher than would be desired.
There is a present need for an improved means in the form of for rendering the exhaust from diesel engines more environmentally benign by the combined use of mechanical devices and fuel additives. In particular, there is a need for improvements which can: in one embodiment, reduce NO.sub.x emissions and particulates simultaneously; and, in another, reduce the temperature necessary for regenerating a particulate trap while also reducing the emissions of carbon monoxide and unburned hydrocarbons.